This invention relates generally to apparatus and methods for securing telephone circuit-connected computers against access by unauthorized calls. More particularly, but not by way of limitation, the invention relates to security apparatus and methods for protectively interfacing a computer system with a telephone circuit, which computer system includes a modem and a computer that normally transmit an audible carrier signal over the telephone circuit as soon as the modem communicates the computer with the telephone circuit in response to an incoming call.
When a computer is connected to a telephone circuit, as is widely done throughout the world today, the computer becomes accessible to unauthorized users, at least some of whom are sometimes referred to as "hackers." The access which these unauthorized users can obtain can have many adverse affects. For example, valuable information stored in the computer can be destroyed, or confidential information stored in the computer can be acquired, or free computing time can be obtained.
Although security measures are sometimes taken to try to avoid unauthorized access, at least one type of these measures can be readily circumvented or solved by a persistent hacker Illustrated in FIG. 1 is a block diagram of what we believe to be a currently typical telephone circuit-connected computer system arrangement of this type which we believe to be readily accessible by an unauthorized user. In this arrangement a telephone circuit 2 is connected to a modem 4 of the computer system. The modem 4 is also connected to a computer 6 which has internal security 8, such as some means for requiring a correct password to be received once the modem 4 has connected the computer to the telephone circuit 2 but before the computer 6 will further communicate with the caller. Prior to any such security being implemented, however, when the correct telephone number of the modem 4 is keyed, the modem 4 communicates the computer 6 with the telephone circuit 2 and an audible carrier signal is returned to the caller.
This typical arrangement and security measure has been found to have a significant shortcoming. An unauthorized user can program a computer (or buy such a program off-the-shelf) to call, in rapid succession, all the 10,000 possible telephone numbers within a given three-digit exchange. Whenever a called number produces an audible carrier signal, the telephone number is logged because this is readily known to be a number at which a computer is connected. The unauthorized user's computer can then redial this number and input different passwords until the correct one is entered and the computer is accessed. Thus, although there has been some measure of security provided within the computer 6, the fact that a particular telephone circuit contains the computer 6 has been readily learned because of the immediate carrier signal transmission which occurs when the modem 4 answers the incoming call.
In view of this shortcoming, there is the need for an apparatus and method which make it more difficult for an unauthorized user to learn which telephone circuits have computers connected to them. That is, there is the need for an apparatus and method to mask or hide that a computer is connected to a particular telephone circuit.